Prior art position detector apparatus have used gratings as an encoder for linear or angular measurements and comprises a common technique used in many commercial devices. One such prior art device is illustrated in a paper authored by myself and S. Holly in the proceedings of SPIE--The International Society For Optical Engineering, Volume 887, as presented Jan. 14, 1988, in Los Angeles, Calif., in the Acquisition, Tracking and Pointing group. The title of the article was, "Development of An Interferometric Encoder For High Resolution Angular Measurements". The prior art technology of this paper as concerns heterodyne detection and other well-known prior art techniques is incorporated by reference.
The grating for many prior art devices is produced holographically by two beams which interfere at an angle of incidence theta (.theta.), on a substrate which retains the image of the laser pattern. The spacing of the grating will be an amount G which equals lambda (.lambda.) divided by (2 times sine of the angle theta) where lambda is the wavelength of the beam. Prior art techniques have compared the position of a reference grating with one being produced at the time of measurement by the laser involved. Data from the two gratings are then compared in a phase measurement type test to provide an indication of position. A device such as a computer can keep track of the phase changes for total number of cycles changed to get absolute position relative to a reference.
While the use of gratings for an encoder for linear or angular measurements has become a common technique used in many commercial devices, all prior devices known to applicant have involved the use of a grating whose spacing is considerably larger than the wavelength of the lightwave comprising the laser beam signal. The prior art approach has used increasingly accurate and accordingly expensive phase detectors to get the position detection accuracies required for some applications. The approach discussed here would use a small wavelength laser to achieve a small grating space thereby relaxing the requirement on phase detection.
Since the present invention involves the discovery that the grating can have a spacing comparable to or less than the wavelength, one aspect of the present invention allows the gratings to be produced by a short wavelength, (more expensive) laser from the fringe effects produced by the interference of two such lasers and then this grating be used with a longer wavelength but less expensive laser in combination with a low cost phase detector to produce the measurement desired.
A further realization in the present invention is that if a diffracted beam is redirected through the grating N times before being detected, the change in phase will be N times the change in phase of a single pass system.
A multipass system using only the multipass concept portion of the present invention is illustrated by an article in Motion magazine, July/August, 1986, authored by Nishimura and Ishizuka.
It is also recognized in the present invention that while there is less power in a diffracted beam of greater than first order, the higher order diffracted beams have greater phase changes than do the lesser order beams and thus, this factor can be used to increase the phase change in the output signal relative the movement of the grating.
All of the above features presented in this concept when combined lessen the requirement for both the cost of the laser (i.e., a low cost laser may be used) and the cost of the phase detector since the phase detector can be of a much lower accuracy than was required in the prior art.
It is thus an object of the present invention to provide an improved motion detector or diffraction decoder apparatus involving high accuracy motion detection without the necessity of high cost components.